Re: [PATCH] RFC: drm: add support for tiled/compressed/etc modifier in addfb2 (v1.5)

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Jan 29, 2015 at 11:43:07AM +0000, Tvrtko Ursulin wrote:
> 
> On 01/29/2015 11:30 AM, Daniel Vetter wrote:
> >On Wed, Jan 28, 2015 at 05:57:56PM +0000, Tvrtko Ursulin wrote:
> >>
> >>On 01/28/2015 05:37 PM, Daniel Vetter wrote:
> >>>From: Rob Clark <robdclark@gmail.com>
> >>>
> >>>In DRM/KMS we are lacking a good way to deal with tiled/compressed
> >>>formats.  Especially in the case of dmabuf/prime buffer sharing, where
> >>>we cannot always rely on under-the-hood flags passed to driver specific
> >>>gem-create ioctl to pass around these extra flags.
> >>>
> >>>The proposal is to add a per-plane format modifier.  This allows to, if
> >>>necessary, use different tiling patters for sub-sampled planes, etc.
> >>>The format modifiers are added at the end of the ioctl struct, so for
> >>>legacy userspace it will be zero padded.
> >>>
> >>>TODO how best to deal with assignment of modifier token values?  The
> >>>rough idea was to namespace things with an 8bit vendor-id, and then
> >>>beyond that it is treated as an opaque value.  But that was a relatively
> >>>arbitrary choice.  There are cases where same tiling pattern and/or
> >>>compression is supported by various different vendors.  So we should
> >>>standardize to use the vendor-id and value of the first one who
> >>>documents the format?
> >>
> >>Maybe:
> >>	__u64 modifier[4];
> >>	__u64 vendor_modifier[4];
> >
> >Seems rendundant since the modifier added in this patch is already vendor
> >specific. Or what exactly are you trying to accomplish here?
> 
> I am trying to avoid packet-in-a-packet (bitmasks) mumbo-jumbo and vendor id
> on the head followed by maybe standardized or maybe vendor specific tag.
> Feels funny. Would it not be simpler to put a struct in there?

The u64 modifier is just an opaque thing, with 8 bit to identifier the
vendor (for easier number management really) and the low 56 bits can be
whatever we want them. On i915 I think we should just enumerate all the
various tiling modes we have. And if the modifiers aren't set we use the
tiling mode of the underlying gem bo. We already have code in place to
guarantee that the underlying bo's tiling can't change as long as there's
a kms fb around, which means all code which checks for tiling can switch
over to these flags.

struct won't work since by definition this is vendor-specific, and every
vendor is slightly insane in a different way.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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